If a concussion leaves someone unconscious for more than a few minutes, the concussion is clearly serious. But sometimes even seemingly mild concussions can do damage.

"A minor hit on the field can take a long time to recover," says Mark Lovell, PhD, founding director of the University of Pittsburgh Medical Center (UPMC) Sports Medicine Concussion Program.

How many concussions are too many? That may be the wrong question.

"It's not as simple as how many concussions someone's had -- it's total brain trauma" that matters, says Boston University neurosurgery professor Robert Cantu, MD, who is a senior advisor to the NFL's Head, Neck, and Spine Committee.

"Linemen who've had almost no concussions have the majority of cases of chronic traumatic encephalopathy, because on every play they get their brains rattled, trying to block with their head," Cantu says.

NFL's Rules

In 2011, the NFL set rules to determine whether an athlete who’s taken a powerful hit and sustained a concussion will be benched or sent back into the game.

The guidelines include checking the player's symptoms, attention, memory, and balance, starting immediately, on the sidelines.

"It incorporates the most important aspects of a focused exam, so that injury is identified, and athletes with concussion and more serious head and spine injury can be removed from play," says Margot Putukian, MD, chair of the return-to-play subcommittee of the NFL's head, neck, and spine committee.